An early Ford Mustang test mule has been spied testing parts likely to be seen on the next-generation 2015 model, a car that will mark the 50th anniversary of the classic Pony car.

The Mustang's next-gen clues are dominated by the spied rear control arms that indicate the mule is running independent rear suspension in favour of the solid rear axle previously used on every 'Stang before it.

The bulging, camouflaged fuel filler door - a common sight on early development mules - indicates a change in the chassis' hard points. Other hints include the A-pillar-mounted gauge pod likely measuring boost pressure or vacuum pressure, and the cut and modified lower-half of the rear bumper surrounding the exhaust.

There also appears to be a structural difference in the roof with the Mustang mule having a large gap on either side of the roof panel that looks as if it's been filled with some sort of filler or putty.

Our spy photographers assured us that while no shots of the front of car were snapped, the mule sported no changes, appearing identical to the current Ford Mustang GT.

The next-generation Mustang is likely to appeal to some with its Evos- and Fusion-based looks, intended to appeal to Generation Y, while others will have to come to terms with the iconic car shying away from its muscle car roots, including the likely shift to a new EcoBoost twin-turbo V6 engine from an old-school V8.